To answer your questions about the points it works like this......2 points for every post and if you post a thread, you get one point for each reply.
To see what the store has to offer check it out right here.
You can get more then just a sig image.....so take a look around. Just for quick information, you need 900 points to get that sig image.
Also, I wouldn't post your picture at the end of every post....I'd just put a link in the sig til you can get the actual picture.
Finally, I think your avatar should be fine. Mine is uploaded from my pc so I don't see how yours will be a problem.